One scientific group in Maramba claims that they have identified a past change in infant feeding practices as the real cause of DM. The Marambian government has asked you to evaluate this admittedly speculative hypothesis. Information on infant feeding practices can be obtained from medical records, through personal interview with mothers, or from historical sales data on formula sales for each of the 20 towns in Maramba. Marketing data indicate that about half of Marambian mothers feed their infants with formula, though consumption varies substantially across towns. The following study designs, based on Maramba's efficient medical record system and the very high willingness of their population to participate in health research, have been proposed:

Study A: Using pediatricians’ records, randomly select 200 children with DM and 800 children without DM; interview their mothers concerning infant feeding history. Compare the infant feeding history among children with and without DM.
Study B: Review pediatricians’ records for a random sample of 1,000 youth age 12-19 years old and record DM status and infant feeding history for each youth.
Study C: Send letters and make follow-up telephone calls to the mothers of 1,000 12-year old boys and girls who have tested negative for diabetes within the past six months. Request the parents’ consent to review their child’s medical records annually until the child reaches age 20. Obtain information on infant feeding status from the medical record and a parental questionnaire.
Study D: Send letters and make follow-up telephone calls to 1,000 mothers of newborn babies asking the mothers to participate in a study in which the investigators will randomly assign mothers to breastfeed or formula feed and will review the annual pediatric visit records for the child through age 20 years.
Study E: Examine the correlation between per capita formula sales and DM prevalence across the 20 towns in Maramba.

1. What is study A?

a. case control
b. cross sectional
c. rct
d. cohort

2. What is study B?

a. case control
b. cross sectional
c. ecological
d. cohort

3. What is study C?

a. case control
b. cross sectional
c. rct
d. cohort

4. What is study D?

a. case control
b. cross sectional
c. rct
d. cohort

5. what is study E? choices: case control, cross sectional, ecological, cohort